Monolayer Semiconductor Superlattices with High Optical Absorption.

Optical absorption plays a central role in optoelectronic and photonic technologies. Strongly absorbing materials are thus needed for efficient and miniaturized devices. A uniform film much thinner than the wavelength can only absorb up to 50% of the incident light when embedded in a symmetric and homogeneous environment. Although deviating from these conditions allows higher absorption, finding the thinnest possible material with the highest intrinsic absorption is still desirable. Here, we demonstrate strong absorption by artificially stacking WS monolayers into superlattices. We compare three simple approaches based on different spacer materials to surpass the peak absorptance of a single WS monolayer, which stands at 16% on ideal substrates. Through direct monolayer stacking without an intentional spacer, we reach an absorptance of 27% for an artificial bilayer, although with limited control over interlayer distance. Using a molecular spacer via spin coating, we demonstrate controllable spacer thickness in a bilayer with 25% absorptance while increasing photoluminescence thanks to doping. Finally, we exploit the atomic layer deposition of alumina spacers to boost the absorptance to 31% for a 4-monolayer superlattice. Our results demonstrate that monolayer superlattices are a powerful platform directly applicable to improve strong light-matter coupling and enhance the performance of nanophotonic devices such as modulators and photodetectors.